Phase change inkjet printers receive phase change ink in a solid form, commonly referred to as ink sticks. Solid ink sticks are loaded into a printer and then melted to produce liquid ink that is used to form images on print media. Phase change inkjet printers form images using either a direct or an offset (sometimes called indirect) print process. In a direct print process, melted ink is jetted directly onto print media to form images. In an offset print process, melted ink is jetted onto a surface of a rotating member, such as the surface of a rotating drum, belt, or band. Print media are moved proximate the surface of the rotating member in synchronization with the ink images formed on the surface. The print media are then pressed against the surface of the rotating member as the media passes through a nip formed between the rotating member and a transfix roller. The ink images are transferred and affixed to the print media by the pressure in the nip.
Offset phase change inkjet printers utilize drum maintenance units (DMUs) to facilitate the transfer of ink images to the print media. A DMU is usually equipped with a reservoir that contains a fixed supply of release agent (e.g., silicon oil), and an applicator for delivering the release agent from the reservoir to the surface of the rotating member. One or more elastomeric metering blades are also used to meter the release agent onto the transfer surface at a desired thickness and to divert excess release agent and un-transferred ink pixels to a reclaim area of the drum maintenance system. The collected release agent is filtered and returned to the reservoir for reuse.
A small amount of release agent is removed from the system with each print. The control system of the printer utilizes a life-sensing process to predict when the supply of release agent is likely to be depleted so an alert can be generated indicating that the DMU is in need of replacement before the supply is exhausted. Volume sensors are impractical so previously known life-sensing processes involve various combinations of open loop print counting and predictions of oil mass remaining in the source following detection of a float sensor reaching a predetermined level in the source. An end-of-life condition is sensed in response to air being detected in the oil intake from the source.
As the supply of release agent in the DMU diminishes, the amount of ink material collected from the rotating member accumulates in the DMU. This ink material can combine with the release agent to form a high viscosity gel-like mixture. As the gel accumulates in the release agent supplied to the applicator, the gel may begin to adhere to the elastomeric blades of the DMU and adversely impact metering performance. In some cases, the gel may contaminate the transfer surface resulting in print defects and inkjet damage. Gel related defects and failures are cumulative and typically occur near the end of the life of the DMU.
Previously known life-sensing processes are helpful in predicting an oil level in the supply of release agent in a DMU. These processes, however, do not take into consideration the factors that lead to gel formation and accumulation in the DMU, and, therefore, are not useful in predicting when a DMU is at risk for gel related failures.